New Species of Ancient 'River' Dolphin Actually Lived in the Ocean

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The fossilized remains of a new species of ancient river dolphin
that lived at least 5.8 million years ago have been found in
Panama, and the discovery could shed light on the evolutionary
history of these freshwater mammals.

Researchers found half a skull, a lower jaw with an almost
complete
set of conical teeth, a right shoulder blade and two small
bones from a flipper. The fossils are estimated to be between 5.8
million and 6.1 million years old, making them from the late
Miocene epoch, researchers said in a new study.

The ancient mammal was discovered on the Caribbean coast of
Panama, at the same site where other marine animal fossils have
been found, which suggests that I. panamensiswas also a
saltwater species, the researchers said.

"We discovered this new fossil in marine rocks, and many of the
features of its skull and jaws point to it having been a marine
inhabitant, like modern oceanic dolphins," study lead author
Nicholas Pyenson, a curator of fossil marine mammals at the
Smithsonian's National Museum of Natural History in Washington,
D.C.,
said in a statement.

But despite dwelling in the salty waters of the Caribbean Sea,
I. panamensis is actually more closely related to
modern-day freshwater river dolphins, the researchers said. In
fact, "Isthminia is actually the closest relative
of the living Amazon river dolphin," study co-author Aaron O'Dea,
a staff scientist at the Smithsonian Tropical Research Institute
in Panama, said in a statement.

Only four species of river dolphins exist today (although one,
the Yangtze river dolphin,
is now likely extinct ), all living in freshwater or coastal
ecosystems. All of these river dolphins moved from marine to
freshwater habitats, developing
broad, paddlelike flippers ; flexible necks; and heads with
particularly long, narrow snouts as they evolved, according to
the study. These adaptations allowed the river dolphins to better
navigate and hunt in winding, silty rivers, the researchers said.

"Many other iconic freshwater species in the Amazon — such as
manatees, turtles and stingrays — have marine ancestors, but
until now, the fossil record of river dolphins in this basin has
not revealed much about their marine ancestry," Pyenson said.
"[I. panamensis] now gives us a clear boundary in
geologic time for understanding when this lineage invaded
Amazonia."

Whales and dolphins evolved from terrestrial ancestors into
marine animals, but river dolphins represent a backward
evolutionary path, moving from oceans inland to freshwater
ecosystems, the researchers said.

"As such, fossil specimens may tell stories not just of the
evolution of these aquatic animals, but also of the changing
geographies and ecosystems of the past," O'Dea said.